Solvent recovery process



United States Patent 3,470,087 SOLVENT RECOVERY PROCESS Donald B. Broughton, Evanston, Ill., assignor to Universal Oil Products Company, Des Plaines, 111., a corporation of Delaware No Drawing. Filed Sept. 11, 1967, Ser. No. 666,971 Int. Cl. (110g 21/22, 25/00; B01j 1/22 U.S. Cl. 208-321 10 Claims ABSTRACT OF THE DISCLOSURE A technique to be integrated into an extraction process designed to recover aromatic hydrocarbons from a mixture thereof with non-aromatic hydrocarbons. The substantially aromatic-free raffinate phase, containing minor quantities of a sulfolane-type solvent characteristically selective for absorbing aromatic hydrocarbons, is passed through activated carbon for adsorption thereon of the solvent. The activated carbon, containing the solvent, is then back-flushed, or washed, with an aromatic hydrocarbon stream to desorb the solvent. The technique is particularly adaptable where a substantially solventfree raflinate is required.

APPLICABILITY OF INVENTION The invention described herein is adaptable for use in a process designed for the separation, and ultimate recovery of aromatic hydrocarbons from a mixture thereof with non-aromatic hydrocarbons, which separation is effected by means of a selective solvent of the sulfolanetype. More specifically, my invention is directed toward the separation and recovery of the solvent, which is characteristically selective for absorbing aromatic hydrocarbons, from the substantially aromatic-free raflinate phase.

The present invention is concerned with an improvement in the type of separation process wherein a mixture of various classes of hydrocarbons is introduced into an extraction zone at an inter-mediate point thereof and is countercurrently contacted therein with a solvent selective for aromatic hydrocarbons. A raflinate phase comprising substantially all of the non-aromatic hydrocarbons in the feed stock and a minor quantity of solvent, is removed from one end portion of the extraction zone, the aromatic solute being subsequently recovered by stripping and fractionating the extract phase.

My invention is applicable for use with any hydrocarbon feed stock having a sulficiently high aromatic concentration to justify the recovery thereof-Le. at least about 25.0% by weight. The overall carbon number range of suitable charge stocks is from about six to about ten. These charge stocks will generally include, in addition to C C and C aromatics, non-aromatics which predominate in C and C paraflins and naphthenes. Exemplary of various sources of suitable charge stocks are the depentanized effluent from a catalytic reforming unit, coke oven by-products, Wash oils, hydrotreated pyrolysis naphtha (the hydrotreating is generally required for olefin saturation and contaminant removal), etc.

Although the method of this invention is applicable to various extraction processes utilizing solvents having a selectivity for aromatics and/ or other select components, it is most advantageously applied where the solvent is a sulfolane-type. By this I mean a solvent having a membered ring, one atom of which is sulfur, the other four being carbon and having two oxygen atoms bonded 3,470,087 Patented Sept. 30, 1969 ice to the sulfur atom. Many of these solvents may be illustrated by the following structural formula:

R2( )H-CHR3 wherein R R R and R are independently selected from the group consisting of a hydrogen atom, an alkyl group having up to 10 carbon atoms, an alkoxy radical having up to 8 carbon atoms and an arylalkyl radical having up to 12 carbon atoms. Other sulfolane-type solvents preferably included within this process are the sulfolenes such as 2-sulfolene or 3-sulfolene which have the following structure:

PRIOR ART Processes for the recovery of aromatic hydrocarbons from hydrocarbon mixtures have been known for many years. These usually involve extractive distillation, or countercurrent liquid-phase contacting of the feed with a solvent selective for aromatics. The prior art also teaches the sulfolane-type solvent for extracting aromatic hydrocarbons from a mixture thereof with non-aromatics. Although sulfolane-type solvents are highly satisfactory as selective solvents for recovering aromatics, their high solubility results in higher concentrations of the heavier non-aromatic components in the extract phase. To displace these non-aromatic components from the extract phase with a light paraffin stream, especially when the non-aromatic components have an appreciable concentration of naphthenes requires a large back-flush stream which must be eventually separated and accordingly involves a large expense. If the feed stock non-aromatic components present in the extract phase are not displaced, an additional burden is placed on the extractive stripper, and some of these components will not be readily stripped out of the aromatic-solvent mixture in the extractive stripper.

The use of a large back-flush stream results in increased quantities of solvent being carried over in the raffinate phase, and to the extent that subsequent processing of the non-aromatic raffinate is precluded. For example, one use for the non-aromatic rafiinate phase resides in the catalytic production of hydrogen. The catalytic compositei.e. a nickel-containing hydrocracking catalystrapidly loses its capability to function acceptably as a result of being poisoned by sulfolane sulfur. Prior art schemes for reducing the concentration of sulfolane 3 sulfur in the ratfinate phase include water-washing in one or more stages. While effecting a reduction in the sulfolane content of the rafiinate, methods such as this fall short of producing an acceptable rafiinate from the standpoint of sufolane sulfur.

OBJECTS AND EMBODIMENTS An object of my invention is the purification of a nonaromatic raffinate containing a sulfolane-type solvent. A corollary objective is to provide a process for recovering sulfolane from the non-aromatic railinate phase of an aromatic extraction process.

A principal object of my invention is, therefore, to increase the purity of the raflinate phase with respect to the concentration of sulfolane sulfur.

Therefore, in a broad embodiment, my inventive concept encompasses a process for purifying a non-aromatic ratfinate stream containing a sulfolane-type solvent characteristically selective for absorbing aromatic hydrocarbons, which process comprises passing said rafiinate stream through activated carbon at a temperature below about 125 F. and recovering a raffinate stream reduced in the concentration of said solvent.

A more limited embodiment involves a cyclic process for the recovery of a sulfolane-type solvent from a nonaromatic raflinate stream, said solvent characteristically selective for absorbing aromatic hydrocarbons, which process comprises the steps of: (a) passing said rafiinate stream through activated carbon at a temperature below about 125 F., adsorbing said solvent onto said activated carbon; (b) discontinuing the fiow of said raflinate through said activated carbon; (c) passing an aromatic hydrocarbon-containing stream through said carbon in reverse flow, with respect to the flow of raffinate, at a temperature above 100 F., desorbing the solvent from said carbon; and, (d) discontinuing the flow of said aromatic-containing stream and re-introducing said raffinate stream through said carbon in reverse flow, with respect to the flow of said aromatic stream.

These and other objects and embodiments will become more apparent from the following detailed description of my invention.

SUMMARY OF INVENTION Activated carbon is a particularly suitable adsorbent for sulfolane as a result of its hydrophobic nature. Water, generally contained in the raflinate phase, is adsorbed only to a minor extent, and will not, therefore, interfere with the adsorption of sulfolane. Other adsorbents such as silica gel or activated alumina are capable of selectively adsorbing sulfolane from a hydrocarbon mixture. However, they rapidly become water-saturated and ineffective for sulfolane removal unless periodically subjected to high temperature drying.

In accordance with my invention, the sulfolane-containing rafiinate phase is passed through a bed of activated carbon at a temperature below about 125 F and preferably in the range of about 70 F. to about 125 F. The liquid hourly space velocity, herein defined as volumes of rafiinate charged per hour per volume of activated carbon, is generally in the range of from 1.0 to about 3.0. When analyses of the recovered clean rafiiuate indicate sulfolane break-throughe.g. more than about 1.0 p.p.m., as sulfurthe flow of raflinate is ceased and an aromatic hydrocarbon-rich stream is introduced in the reverse direction.

In view of the fact that the solvent recovery system described herein is designed for integration into a sulfolane extraction process, the fresh, aromatic-containing charge stock to the extraction process conveniently serves as the desorbent to remove sulfolane from the activated carbon. The aromatic hydrocarbon stream is passed through the carbon in reverse flow, with respect to the flow of raflinate, and at a liquid hourly space velocity of from about 3.0 to about 5.0. The temperature of the aromatic stream employed for elution of the sufolane is generally above F., and preferably in the range of from about 100 F. to about 175 F. Since the affinity of activated carbon for sulfolane is lower at higher temperatures, the adsorption is effected at a lower temperature than elution.

It will be recognized that the present concept readily lends itself to a cyclic, swing-bed system wherein one bed of activated carbon is removing sulfolane from the raflinate while a second, distinctly individual bed is being washed with the aromatic feed stock. In a commerciallyscaled system, the ratfinate and aromatic feed stock are alternately charged to the activated carbon bed in reverse directions. To permit purging of the bed, the efiluent would be directed to the extractor feed or raffinate storage on a delayed time cycle.

Example This example is present for illustrative purposes only, and is not intended to limit my invention beyond the scope and spirit of the appended claims.

A packed bed of Pittsburgh BPL carbon, 30-50 mesh was conditioned by contact with an aromatic-rich extractor feed stock; 11.2 grams (24.0 cc.) of carbon were employed. The aromatic-rich stock contained 9.9% by weight of benzene, 25.3% toluene and 25.7% xylene, the remainder being principally C to C non-aromatics. Conditioning was effected at a temperature of 932 F. for a period of 75 minutes, during which time 101.3 cc. of feed stock passed through the activated carbon at an average LHSV of 3.36.

A simulated raffinate stream was prepared to contain p.p.m. (by weight) of sulfolane and a total of 7.1% by weight of aromatics. In addition, the rafiinate stream contained about 33 p.p.m. of Water. The rafi'inate contacted the carbon in upward flow at a temperature of about 932 F. and an LHSV of about 2.05. The results are present in the following Table I:

TABLE I-RAFFINATE PROCESSING The next analyses of the rafiinate efiluent were made after 1039 minutes, processing 856 cc. of charge. They indicated 26 p.p.m. of water, 68.0 p.p.m. of sulfolane and 7.1 wt. percent aromatics. Considering 1.2 p.p.m. of sulfolane to be acceptable, the break-through occurred after processing 733 cc. of raflinate, or about 30.5 volumes per volumes of activated carbon.

The spent carbon was washed in downward flow, at a temperature of 152.6 F. and an LHSV of about 4.1, with a typical aromatic-containing extractor feed stock which comprises 9.6 wt. percent benzene, 23.9% toluene and 23.8% xylenes. The results are present in the following Table 11:

TABLE II-EXTRACTOR FEED WASHING Time in Total sulfolane, Aromatics, Mlnutes Charge, cc. p.p.m. wt. percent level of 93.2" F. Results are present in the following Table III:

TABLE III-RAFFINATE PROCESSING For this portion of the cyclic operation, th sulfolane break-through occurred at about 31.5 volumes of raflinate per volume of activated carbon.

The foregoing specification, and particularly the example, clearly indicates the means by which my invention is utilized to recover traces of solvent while simultaneously purifying a raifinate stream to the extent that the same is suitable for subsequent processing.

I claim as my invention:

1. A process for purifying a non-aromatic hydrocarbon raffinate stream containing a sulfolane-type solvent characteristically selective for absorbing aromatic hydrocarbons which comprises passing said raflinate stream through activated carbon at a temperature below about 125 F. and adsorbing said solvent onto said activated carbon, and recovering a raflinate stream reduced in the concentration of said solvent.

2. The process of claim 1 further characterized in that said solvent is selected from the group consisting of 2- sulfolene and 3-sulfolene.

3. The process of claim 1 further characterized in that said solvent is a sulfolane having the general formula:

where R R R and R are independently selected from the group consisting of a hydrogen atom, an alkyl group having up to ten carbon atoms, an alkoxy radical leaving up to eight carbon atoms and an arylalkyl radical having up to twelve carbon atoms.

4. The process of claim 1 further characterized in that said raflinate stream contacts said activated carbon at a temperature in the range of from 70 F. to about 125 F.

5. The process of claim 1 further characterized in that the liquid hourly space velocity of said raflinate stream through said activated carbon is in the range of from 1.0 to about 3.0.

6. A cyclic process for the recovery of a sulfolane-type solvent from a non-aromatic hydrocarbon raffinate stream,

said solvent characteristically selective for absorbing aromatic hydrocarbons, which process comprises the steps of:

(a) passing said raflinate stream through activated carbon at a temperature below about F., adsorbing said solvent onto said activated carbon;

(b) discontinuing the flow of said raffinate through said activated carbon;

(0) passing an aromatic hydrocarbon-containing stream through said carbon in reverse flow, with respect to the flow of raffinate, at a temperature above 100 F., desorbing the solvent from said carbon; and,

(d) discontinuing the flow of said aromatic-containing stream and re-introducing said rafiinate stream through said carbon in reverse flow, with respect to the flow of said aromatic stream.

7. The process of claim 6 further characterized in that said aromatic hydrocarbon-containing stream passes through said activated carbon at a temperature in the range of from about 100 F. to about F.

8. The process of claim 6 further characterized in that the liquid hourly space velocity of said aromatic stream through the activated carbon is in the range of from about 3.0 to about 5.0.

9. The process of claim 6 further characterized in that said raflinate contacts said activated carbon at a temperature below that at which said aromatic stream contacts said activated carbon.

10. The process of claim 6 further characterized in that said solvent is a sulfolane having the general formula:

where R R R and R are independently selected from the group consisting of a hydrogen atom, an alkyl group having up to ten carbon atoms, an alkoxy radical leaving up to eight carbon atoms and an arylalkyl radical having up to twelve carbon atoms.

References Cited UNITED STATES PATENTS 2,379,654 7/ 1945 Royer 208-250 2,644,018 6/1953 Harper 260676 2,700,690 1/1955 Mottern 208250 2,937,139 5/1960 Hoyer 208321 2,967,148 l/l961 Karnofsky 260-676 HERBERT LEVINE, Primary Examiner US. Cl. X.R. 208-250, 307, 325 

